1. Field of the Invention
This invention relates to non-azide gas generant, or propellant compositions, generally in pellet or tablet form, which are burned to provide primarily nitrogen gas to inflate automobile air bag restraint systems. More particularly this invention relates to improved propellant compositions including an oxidizer and a novel non-azide fuel for producing the gas comprising a transition metal complex of an aminoarazole.
Though the gas generant or propellant compositions of this invention are especially designed and suited for creating a nitrogen-containing gas for inflating passive restraint vehicle crash bags, they would function equally well in other less severe inflation applications, such as aircraft slides and inflatable boats; and, more generally, would find utility for any use where a low temperature, non-toxic gas is needed, such as for a variety of pressurization and purging applications, as in fuel and oxidizer tanks in rocket motors; for various portable and military equipment and operations where a storable source of gas is needed.
2. Description of the Prior Art
Automobile air bag systems have been developed to protect the occupant of a vehicle, in the event of a collision, by rapidly inflating a cushion or bag between the vehicle occupant and the interior of the vehicle. The inflated air bag absorbs the occupants' energy to provide a gradual, controlled ride down, and provides a cushion to distribute body loads and keep the occupant from impacting the hard surfaces of the vehicle interior.
The most common air bag systems presently in use include an on-board collision sensor, an inflator, and a collapsed, inflatable bag connected to the gas outlet of the inflator. The inflator typically has a metal housing which contains an electrically initiated igniter, a gas generant composition, for example, in pellet or tablet form, and a gas filtering system. Before it is deployed, the collapsed bag is stored behind a protective cover in the steering wheel (for a driver protection system) or in the instrument panel (for a passenger system) of the vehicle. When the sensor determines that the vehicle is involved in a collision, it sends an electrical signal to the igniter, which ignites the gas generant composition. The gas generant composition burns, generating a large volume of relatively cool gaseous combustion products in a very short time. The combustion products are contained and directed through the filtering system and into the bag by the inflator housing. The filtering system retains all solid and liquid combustion products within the inflator and cools the generated gas to a temperature tolerable to the vehicle passenger. The bag breaks out of its protective cover and inflates when filled with the filtered combustion products emerging from the gas outlet of the inflator. See, for example, U.S. Pat. No. 4,296,084.
The requirements of a gas generant suitable for use in an automobile air bag are very demanding. The gas generant must burn very fast to inflate the air bag, for example, in about 30 milliseconds or less, but the burn rate must be stable, controllable and reproducible to ensure bag deployment and inflation in a manner which does not cause injury to the vehicle occupants or damages to the bag.
The gas generant must be extremely reliable during the life of the vehicle (ten years or more). Ignition must be certain, and the burn rate of the gas generant composition must remain constant despite extensive exposure of the composition to vibration and a wide range of temperatures. The gas generant is protected from moisture when sealed in the inflator, but should still be relatively insensitive to moisture to minimize problems during manufacture and storage of the gas generant and assembly of the inflator, and to ensure reliability during the life of the air bag system.
The gas generant must efficiently produce cool, non-toxic, non-corrosive gas which is easily filtered to remove solid or liquid particles, and thus to preclude injury to the vehicle occupants and damage to the bag.
It follows then that the most desirable atmosphere inside an inflated crash bag would correspond in composition to the air outside it. This has thus far proven impractical to attain. The next best solution is inflation with a physiologically inert or at least innocuous gas. The one gas which possesses the required characteristics and which has proven to be the most practical is nitrogen.
The most sucessful to date of the prior art solid gas generants which produce nitrogen that are capable of sustained combustion have been based upon the decomposition of compounds of alkali metal, alkaline earth metal and aluminum derivatives of hydrazoic acid, especially sodium azide. Such azide-containing gas generants are disclosed in, for example, U.S. Pat. Nos. 2,981,616; 3,814,694; 4,203,787 and 4,547,235.
There are some disadvantages, however, to the use of azides in gas generant compositions used for inflating air bag systems. For instance, sodium azide is a Class B poison and is a highly toxic material. It is easily hydrolyzed, forming hydrazoic acid which is not only a highly toxic and explosive gas, but also readily reacts with heavy metals such as copper, lead, etc. to form extremely sensitive solids that are subject to unexpected ignition or detonation. Especially careful handling in the manufacture, storage and eventual disposal of such materials is required to safely handle them and the azide-containing gas generants prepared from them.
A number of approaches to a non-azide nitrogen gas generant have been investigated in the prior art, as disclosed, for example in U.S. Pat. Nos. 3,004,959; 3,055,911; 3,348,985; 3,719,604 and 3,909,322. Many of the prior art nitrogen gas generants that have been reported are based upon nitrogen-containing compounds such as those derived from the various hydroxylamine acid and hydroxylamine derivatives, while others consist of various polymeric binders, hydrocarbons and carbohydrates which are oxidized to produce non-corrosive and, often termed, "non-toxic" gases. The gas products from these compositions, however, contain unacceptably high levels of carbon dioxide, carbon monoxide and water for use in automobile air bag applications where the possibility exists that the occupant may breathe, even for short periods of time, high concentrations of the gases produced from the gas generant. Thus, these compositions do not meet the present requirements that the combustion products meet industrial standards for toxic and other gases such as carbon monoxide, carbon dioxide, etc.
Non-azide materials, such as tetrazole derivatives have also been used in gas generant and explosive compositions. For example, U.S. Pat. No. 1,511,771 discloses that alkali, alkaline earth and heavy metal salts of tetrazole, tetrazoleazoimid, diazotetrazoleimid, azotetrazole, oxyazotetrazole, diazoaminotetrazole, diazotetrazole, bistetrazole, phenyltetrazole carbon acid, methyl mercaptotetrazole, substituted dioxytetrazoles, phenethenyldioxytetrazol, .beta.-naphthenyldioxytetrazol, phenylglcyolendroxytetrazole, benzenyldioxytetrazol, meta-nitro-benzenyldioxytetrazol, and para-tolenyldioxytetrazole are useful in explosive compositions.
U.S. Pat. No. 3,055,911 discloses vinyltetrazoles which can be polymerized to provide polymers having large percentages of nitrogen. These polymers are useful as polymeric fuel matrices and binders for composite propellants and explosives.
U.S. Pat. No. 3,171,249 discloses hydrazine-based rocket fuels which contain aminotetrazole or its salts. The addition of aminotetrazole to the rocket fuel is said to make the fuel storable and have a lower freezing point.
U.S. Pat. No. 3,348,985 discloses gas generating compositions containing a mixture of ammonium nitrate and aminotetrazole. The gas generants are said to increase the useable and effective gas volume produced by the generant.
U.S. Pat. No. 3,468,730 discloses propellants containing a tetrazole derivative such as 5-aminotetrazole, guanylamino-5-tetrazole or 1-guanyl-3-tetrazolyl-guanidine. The propellant also contains an oxidizer such as barium nitrate, potassium dichromate, potassium nitrate, lead dioxide, copper oxide and manganese dioxide.
U.S. Pat. No. 3,719,604 relates to gas generating compositions containing aminoguanidine salts of azotetrazole or of ditetrazole. These compositions are said to generate large quantities of gas, but without explosive spontaneous decomposition.
U.S. Pat. No. 3,734,789 discloses gas generating solid composite propellants containing 5-aminotetrazole nitrate as the oxidant component. Likewise, U.S. Pat. No. 3,739,574 discloses a gas generator which may contain 5-aminotetrazole.
U.S. Pat. No. 3,873,477 discloses 5-aryltetrazole metal salts of zinc, barium, calcium, lead and aluminum which are useful as blowing agents in high-temperature processing of such polymers as polycarbonates and polysulfone resins.
U.S. Pat. No. 3,898,112 discloses a solid, gas generating propellant based on 5-aminotetrazole nitrate as the oxidant. Solid gas generating compositions are also disclosed in U.S. Pat. No. 3,909,322 which contains nitroaminotetrazole salts such as guanidinium 5-nitroaminotetrazole, ammonium 5-nitroaminotetrazole and hydrazinium 5-nitroaminotetrazole. The composition also contains an oxidant which can, for example, be 5-aminotetrazole nitrate.
U.S. Pat. No. 3,912,561 relates to a gas generating composition comprising an azide fuel, an oxidant, and a nitrogenous compound selected from aminotetrazole, aminotetrazole hydrate, azodicarbonamide and azotetrazole. The composition is said to produce a high yield of substantially non-toxic gas at moderate temperature and within a short period of time.
U.S. Pat. No. 3,954,528 discloses gas generants containing triaminoguanidine nitrate and an oxidant. One example of the oxidant is 5-aminotetrazole nitrate.
U.S. Pat. No. 4,369,079 discloses solid, non-azide nitrogen gas generant compositions which contain a metal salt of a non-hydrogen containing tetrazole compound selected from alkali metal salts and alkaline earth metal salts of, e.g., bitetrazole or azotetrazole compounds such as aminotetrazole, bistetrazoletetrazine, tetrazole, polyhydrazides or poly azo-alkyl.
Finally, U.S. Pat. No. 4,370,181 relates to solid, non-azide gas generating compositions which contain a non-hydrogen containing metal salt of 5,5'-bitetrazole, including the disodium, dipotassium and calcium salts of bitetrazole.
In contrast to the above discussed prior art, it has now been discovered that improved non-azide, gas generating compositions can be made using transition metal complexes of aminoarazoles.